Messages in this thread | | | Date | Fri, 11 Aug 2023 16:34:10 -0700 | Subject | Re: [PATCH bpf-next] bpf: Support default .validate() and .update() behavior for struct_ops links | From | Martin KaFai Lau <> |
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On 8/11/23 4:12 PM, Kui-Feng Lee wrote: > > > On 8/11/23 15:49, Martin KaFai Lau wrote: >> On 8/11/23 1:19 PM, David Vernet wrote: >>> On Fri, Aug 11, 2023 at 10:35:03AM -0700, Martin KaFai Lau wrote: >>>> On 8/10/23 4:15 PM, Stanislav Fomichev wrote: >>>>> On 08/10, David Vernet wrote: >>>>>> On Thu, Aug 10, 2023 at 03:46:18PM -0700, Stanislav Fomichev wrote: >>>>>>> On 08/10, David Vernet wrote: >>>>>>>> Currently, if a struct_ops map is loaded with BPF_F_LINK, it must also >>>>>>>> define the .validate() and .update() callbacks in its corresponding >>>>>>>> struct bpf_struct_ops in the kernel. Enabling struct_ops link is useful >>>>>>>> in its own right to ensure that the map is unloaded if an application >>>>>>>> crashes. For example, with sched_ext, we want to automatically unload >>>>>>>> the host-wide scheduler if the application crashes. We would likely >>>>>>>> never support updating elements of a sched_ext struct_ops map, so we'd >>>>>>>> have to implement these callbacks showing that they _can't_ support >>>>>>>> element updates just to benefit from the basic lifetime management of >>>>>>>> struct_ops links. >>>>>>>> >>>>>>>> Let's enable struct_ops maps to work with BPF_F_LINK even if they >>>>>>>> haven't defined these callbacks, by assuming that a struct_ops map >>>>>>>> element cannot be updated by default. >>>>>>> >>>>>>> Any reason this is not part of sched_ext series? As you mention, >>>>>>> we don't seem to have such users in the three? >>>>>> >>>>>> Hi Stanislav, >>>>>> >>>>>> The sched_ext series [0] implements these callbacks. See >>>>>> bpf_scx_update() and bpf_scx_validate(). >>>>>> >>>>>> [0]: https://lore.kernel.org/all/20230711011412.100319-13-tj@kernel.org/ >>>>>> >>>>>> We could add this into that series and remove those callbacks, but this >>>>>> patch is fixing a UX / API issue with struct_ops links that's not really >>>>>> relevant to sched_ext. I don't think there's any reason to couple >>>>>> updating struct_ops map elements with allowing the kernel to manage the >>>>>> lifetime of struct_ops maps -- just because we only have 1 (non-test) >>>> >>>> Agree the link-update does not necessarily couple with link-creation, so >>>> removing 'link' update function enforcement is ok. The intention was to >>>> avoid the struct_ops link inconsistent experience (one struct_ops link >>>> support update and another struct_ops link does not) because consistency was >>>> one of the reason for the true kernel backed link support that Kui-Feng did. >>>> tcp-cc is the only one for now in struct_ops and it can support update, so >>>> the enforcement is here. I can see Stan's point that removing it now looks >>>> immature before a struct_ops landed in the kernel showing it does not make >>>> sense or very hard to support 'link' update. However, the scx patch set has >>>> shown this point, so I think it is good enough. >>> >>> Sorry for sending v2 of the patch a bit prematurely. Should have let you >>> weigh in first. >>> >>>> For 'validate', it is not related a 'link' update. It is for the struct_ops >>>> 'map' update. If the loaded struct_ops map is invalid, it will end up having >>>> a useless struct_ops map and no link can be created from it. I can see some >>> >>> To be honest I'm actually not sure I understand why .validate() is only >>> called for when BPF_F_LINK is specified. Is it because it could break >> >> Regardless '.validate' must be enforced or not, the ->validate() should be >> called for the non BPF_F_LINK case also during map update. This should be fixed. > > For the case of the TCP congestion control, its validation function is > called by the implementations of ->validate() and ->reg(). I mean it > expects ->reg() to do validation as well.
Right, for tcp-cc, the reg is doing the validation because it is how the kernel tcp-cc module is done.
For newer subsystem supporting struct_ops, it should expect the validation is done in the .validate alone.
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